1. Field of the Invention
The present invention relates to an actuator assembly for a magnetic disk drive.
2. Description of the Related Art
In recent years, size reduction and capacity enlargement have been desired in a magnetic disk drive as a kind of external storage for a computer. In association with the size reduction of the magnetic disk drive, an actuator capable of positioning a head with high precision is necessary.
In a magnetic disk drive for a computer, a flying magnetic head is usually used to avoid damage to the head and a magnetic disk due to contact with each other. That is, while the magnetic disk is being rotated at a high speed, a head flying force generated by an air flow due to high-speed rotation of the disk balances a force of a spring for pressing the head on the disk, so that the head is kept at a microscopic flying height from the disk to perform read/write of data on the disk.
In this type of magnetic disk drive, the distance between the head and the disk has a large influence on the data read/write performance of the head. Accordingly, an actuator capable of positioning the head with high precision is required. An example of means for positioning the actuator with high precision is such that bearings for supporting the actuator are constructed as a unit to rotatably support the actuator by the use of such a bearing unit.
The bearing unit is press-fitted within a substantially cylindrical bore formed in an actuator block. The bore has a sectional shape as formed by cutting away a part from a complete round. The bearing unit is in plural-point contact with the inner wall surface of the actuator block defining the bore by screws retracting the bearing unit. This structure prevents distortion of the actuator due to a difference in the coefficient of linear expansion between the actuator and the bearing unit.
In paying attention to a rotational direction of the actuator about the axis of rotation thereof, it is important to consider how to reduce the torque generated in the actuator due to the application of external acceleration, in order to improve the performances of shock resistance and vibration resistance. If the center of rotation of the actuator is not aligned with the center of gravity of the actuator, torque is generated in the actuator upon application of external acceleration. This torque is proportional to the amount of misalignment of the center of gravity, and causes offtrack of the head during the operation of the disk drive. Further, misalignment of the center of gravity operates as a force of canceling a parked state of the actuator in the inoperative condition of the disk drive.
Further, in paying attention to a vertical direction of the actuator, it is important to consider how to suppress vibrations in the seek operation, so as to improve the seek performance. If the rotational force created by the voice coil motor (VCM), which rotates the actuator assembly about a rotational axis, is not aligned with the center of gravity of the actuator, the vibrations generated in the actuator in the seek operation become larger. As a result, there is a problem of increased access time.
For these reasons, it is necessary to reduce the imbalance of the actuator in both the rotational direction and the vertical direction thereof, in order to improve the performance of the magnetic disk drive. If the imbalance of the actuator remains, the performances of shock resistance and vibration resistance are reduced, and settling time is increased because of residual vibrations in the seek operation.
Conventionally, a balancer is added to the actuator to cancel the imbalance of the actuator. However, the addition of the balancer to the actuator causes an increase in inertia of the actuator, resulting in a deterioration in acceleration performance of the actuator. Furthermore, the size reduction of the disk drive makes it difficult to ensure a space for mounting the balancer.